Metal ingot mold with protective coating



United States Patent 3,396,935 METAL INGOT MOLD WITH PROTECTIVE COATINGWilliam T. Snyder, Flossmoor, Ill., assignor to Nalco Chemical Company,Chicago, Ill., a corporation of Delaware No Drawing. Originalapplication Aug. 27, 1965, Ser. No. 483,301. Divided and thisapplication Aug. 10, 1967, Ser. No. 659,621

2 Claims. (Cl. 249174) ABSTRACT OF THE DISCLOSURE An improved metal moldfor casting ingots which has its base member coated with a thinprotective film of refractory material.

This application is a division of my copending application, Ser. No.483,301, now Patent No. 3,357,487, filed Aug. 27, 1965.

INTRODUCTION All metal ingots are cast from molds. One popular type is abig end down mold. These molds rest on bases commonly known as stools.The stools are merely large, normally rectangular, flat slabs of metal,commonly made of cast iron, which are used as support for the mold sidesand also, of course, form the bottom portion of the mold. The moldinside surfaces generally taper up in diameter from bottom to top.

Various problems commonly occur in use of these molds and particularlywith respect to the surface of their base portions. First, theunprotected metal surface quickly erodes and pits from the action ofmolten metals which are cascaded upon their surface. Large gouges in thebase portions are produced due to the force and high temperaturedeveloped by the flowing molten metal which contacts the surface of thestool. Since many molds ar generally approximately 5-10 feet in height,the metal must be poured from a height at least equal to that distanceand quite often is poured from even greater heights. A considerablepressure head is also developed due to the mass of molten metal. Thus,the hot molten metal easily gouges gaping depressions in the basemembers under such force and at a temperature greater than theliquefaction temperature of the molten metal.

The molten metal upon solidification to an ingot thereby has a bottomform conforming to the undesirable eroded surface configuration of thestool or base member of the mold. Thus, a considerable amount of theingot, when withdrawn from the mold and subsequently processed intoslabs or blooms, is lost through a cropping of the irregularly formedend of the slab. This, of course, is highly undesirable, since itresults in increase in scrap, which must be subsequently reprocessed.Thus, there is an overall reduction in yield.

Another extremely serious and costly problem results after the ingot ina big-end-down mold has solidified to a point where it can be removedfrom both the mold sides and its base platform member or stool. In manycases, if the surface of the stool is unprotected, or inadequatelyprotected, and erosion occurs as described above, the ingot has agreater tendency to remain tightly adherent to the stool. Thus, afterthe mold sides are removed from around the ingot, which process cannormally be efficiently achieved with a minimal film of coating selectedfrom a variety of coating agents, the ingot must be forcibly removedfrom the stool. This is normally achieved by raising both ingot andadherent stool, and thrusting them against some other larger objectwhereby the ingot is jarred loose. In many cases, the stool and ingotare merely dropped on the floor from some suitable height. In

such a situation, the stool is often broken into two or more smallerpieces and cannot be subsequent reused in casting other ingots. Again,replacement cost of these stools is high, making this aspect of theoverall casting process somewhat disadvantageous. The same problemexists with respect to closed end big-end-up molds wherein sticking ofingots particularly occurs at their base portion, New molds of this typeare especially vulnerable to sticking due to their smooth surfaceunprotected by any layers of metal oxides or scale. A tightmetal-to-metal bond between mold bottoms and ingots then occur.

Cracking of molds and particularly their base portions due to the abovediscussed rough handling occasioned by stickers between the baseportions and ingots is also enhanced by thermal shock during ingotformation. Unprotected or inadequately protected bottom surfaces ofmolds are especially suceptible to such destructive shock.

Many prior art coating materials have failed to give adequate protectionto the mold base members, and in some cases caused ancillary processdifficulties. For example, some inferior coatings were washed off thesurface of the base portion of the mold and were thereby included as anunwanted impurity in the ingots. Such inclusion oftentimes deleteriouslyaffected desirable metal properties.

US. Patents 3,184,813 and 3,184,815 described excellent methods ofcombating the above described arduous problems. However, it would be offurther benefit to the art if an additional method of stool coating werediscovered which possessed added advantages of lower cost as well asenhanced efficiency. If the above described stools of big-end-down moldsor base members of big-end-up molds could be protected from erosion bycascading molten metal in a more economical operation, such processwould find ready acceptance in the art. Likewise, if the problem ofadherence between a mold base member and subsequently formed ingot couldalso be overcome in a simple, efiicient and economical method, theoverall casting process would be materially benefited.

OBJECTS It therefore becomes an object of the invention to provide amethod of casting metal ingots into metal molds whereby adherence of thebase member of the mold to the formed ingot and erosion of the same basemember during ingot formation are substantially prevented.

A specific object of the invention is to inhibit such erosion andadherence by applying a film of coating material to the surface of thebase member whereby these problems are over-come.

Yet another object is to provide metal molds used for casting ingotswhich have at least the base member of the mold protected by a film ofmaterial so that the above described problems of erosion of the basemember and ad herence of same to metal ingots no longer exist to anyappreciable degree.

Other objects will appear hereinafter.

THE INVENTION In accordance with the invention a method of casting metalobjects from metal molds has been discovered, whereby adherence of thebase member of those molds to ingots formed therefrom, and erosion ofthese same base members during such ingot formation are substantiallyinhibited.

In its broadest aspects, the invention comprises the steps of applying aslurry consisting of a binder and refractory to the surface of the basemember of a metal mold. In order to best achieve dual purposes ofprevention of erosion of base member and non-adherence to formed ingots,the slurry should consist of at least one refractory of the followingtypes: vitreous siilca, crystalline silica, magnesium silicates,aluminum silicates, alumina, graphite,

zirconium silicates and clay. These materials are all wellknownsubstances and are all commercially available. Typical aluminumsilicates, for example, may include mica, a laminated type of aluminumsilicate and mulor even lower. The coating becomes completely resistantto subsequent contact with water and stays tightly adherent to the baseportions of the mold even under such washings.

lite, an orthorhombic aluminum silicate available from 5 After coatingoperations, molten metal is poured into the Island of Mull or othersources or artificially made the mold, all-owed to solidify into aningot and the ingot by heating andalusite, sillimanite or kyanite.Excelis then separated from mold surfaces. The invention i not lentmagnesium silicates are forsterite or talc, while a limited to use withspecific mold sides or any particular useful zirconium silicate iszircon. A typical crystalline metal mold bottom or to use with anyparticular molten silica is quartz. The most preferred refractory,discussed metal. However, it has found specially preferred use in inmore detail hereinafter, is vitreous silica. The binder coating castiron metal base members for molds which used with any one or more of theabove refractories should are used in forming ingots of steel. comprisecolloidal silica sol and a silicic acid sol, both One of the bindercomponents used to form the slurries of which will be fully describedhereinafter. of the invention is a colloidal silica sol. These are well-The slurry is provided in an amount adequate to form a known materialsand are commercially available from sevcoating of sufficient thickess toprevent the above-meneral sources of supply. A typical group ofcommercially tioned adherence and erosion from occurring. After theavailable silica sols that may be used in the practice of slurry isallowed to dry with or without application of heat, the invention arethose silica sols sold under the name whereby the liquid phase is drivenfrom the surface of the Nalcote. Silica sols of this type are describedbelow in base member, leaving a thin film of solid refractory coat-Table I.

TABLE I Silica Sol. I II III IV V VI Percent colloidal Silica as S102 15-3 21-22 -50 35 pH 8.6 10.2 8.6 3.7 9.0 3.5 Viscosity at 77 F., cps. 5 55 10 20-30 6. 5 Specific gravity at 68 F 1. 09 1.205 1.255 1.06 1.385 1.255 Average surface area, m? per gram of Si 330-430 190-270 135-100135-190 120-150 135-190 Average particle Size, millimicrons 7. 9 11-1610-22 16-22 20-25 10-22 Density, lbs/gallon at 08 F 9.1 10.0 10.5 8.811.6 10.5 NazO, percent 0. 04 0. 40 0.10 0.05 0. 30 0.01

ing, the molten metal is thereafter poured intothe mold and on top ofthe now coated base member. The solidified ingot is then removed fromthe coated base member and mold sides.

The slurry coating is simply applied to the stool or bottom of the moldby a wide variety of methods. For example, ithe slurry may be applied byflowing it over the stool, by spray techniques, by coating the stoolwith some type of applicator, etc. Spray application is believed to bethe most efficient and practical way of slurry application. The slurrymay be applied to the stool portion of the molds before or after themetal mold sides are placed thereon.

Effecting removal of the liquid phase of the slurry from the solidrefractory material may likewise be carried out in a variety of methods.'For example, the bottom portion of the mold may be coated with theslurry and allowed to dry gradually. Another method of laying down athin protective coating is to apply the slurry to an already heatedstool or mold having an integral base portion. This is particularlypreferred in that the stools and their metal mold sides or big-end-upmolds are generally already hot before introduction of the molten metaldue to the residual heat from the previous casting run, and in such amethod drying time is a very minimum period.

Another way of applying the protective film is to coat the base member,and then heat it as slowly or rapidly as desired to drive off the liquidportion of the slurry. In any event all that is necessary is that thecoating be laid down and dried in some manner after the slurry has beenapplied to the base member.

In the most preferred method the coating slurry is applied to stools ormold base members having a temperature ranging from that of roomtemperature to 1200 F., and more preferably from 200 F. to 800 F. Mostpreferably, best adherence of solid coating to stools is achieved byslurry application to the stools :at a temperature range.

of 200-500" F. For best results, it has been determined that filmsshould measure in thickness from 0.01" to 3" and most preferably from0.01" to A.

It is believed that the excellent coating success achieved by use of theabove described slurries is due to their ability to form a strongceramic coating even when aflixed to the base members of the mold atrelatively low temperatures, at least under mill conditions, of sayabout 500 F.

Other silica sols that may be used in addition to those above, may beprepared by using several well-known conventional techniques. Perhapsthe mos-t convenient method of making aqueous colloidal silica sols isdescribed in Bird, US. Patent 2,224,355, wherein a dilute solution of analkali metal silicate is passed in contact with a cation exchange resinin hydrogen form, whereby the silicate is converted to a dilute aqueoussilicic acid sol. The dilute silicic acid sol may be both converted to asilica sol and concentrated to solids concentrations which are moreeconomically usable from the standpoint of shipping costs and ultimateprocess use, by employing the techniques described in either Bechtold etal. U.S. Patent 2,574,902; Bragg et al., US. Patent 2,680,720; or Parmaet a1. U.S. Patent 2,601,235. Another type of silica sol which may beused in the practices of the invention is described in the specificationof Reuter, US. Patent 2,856,302. While aqueous colloidal silica sols maybe used, it will be understood that other forms of colloidal silica maybe employed, such as for instance, sols which contain a major portion ofpolar organic solvents. Said sols may be generically referred to asorgano sols, and are typified by the sols described in Marshall US.Patent 2,386,247. It is only necessary that the silica particles usedcan be dispersed colloidally in a hydrophilic substance, such as wateror lower alkyl alcohols and other organic compounds possessingrelatively high dielectric constants.

In some instances mixtures of water and organic substances compatiblewith water may be employed as suspending media for the colloidal silicaparticles. Particularly preferred organic substancesare those whichlower the freezing point of pure aqueous sols by their admixture withthese aqueous silica sols. Such final product sols then are especiallyuseful during the colder months of the year when they must be storedand/or used at relatively low temperature. Amines such as morpholine,diethyl amine, etc., and polyhydroxy organics as ethylene glycol,glycerine, etc., are preferred materials in making up silica solscontaining these substances as sole silica suspending media or as aportion of a mixture additionally containing water. A preferred sol,winterized against freezing contains 5-50 parts by weight of polyhydroxycompound such as ethylene glycol, 20-85 parts by weight of water and10-60 parts by weight of silica.

Regardless of the method employed to produce the colloidal silica solcontaining water, polar organic liquids or mixtures of these substancesas a continuous suspending phase, it is desirable that said sols containsilica particles which are dense, amorphous, and have an averageparticle diameter which does not exceed 150 millimicrons and is greaterthan millimicrons. As evidenced by a reading of Table I, all the silicasols contemplated as starting materials have an average particle sizediameter well below 150 millimicrons. Preferably, the starting silicasols have an average particle size diameter of from -50 millimicrons.The silica concentration in the sols may be between 0.1% and 60% byweight silica expressed as SiO More preferred sols contain from 3.0 to60% by weight of silica and most preferably 10'60% by weight. Themolecular weight of the silica particles is in excess of 200,000 and mayrange as high as several million.

Other sols which may be employed as binders for the silica refractoryare those known as salt-free silica sols. These are particularlypreferred when the suspension media of the silica particles in thebinder itself is solely a polar organic liquid or a mixture of water andpolar organic liquid. Since many of the above described sols usuallycontain alkali metal compounds as stabilizers, they are generally notcompatible with organic systems due to the fact that the salts presentin the aqueous sol cause gelation or precipitation of the silicaparticles when the aqueous phases are exchanged for polar organicsolvents. This can be avoided by use of salt-free aqueous silica sols asstarting materials in preparation of pure organosols or mixtures ofwater and organic as silica carriers. In order to avoid this gelationelfect, it is necessary that the causative cations be removed from thesurface of the colloidally dispersed silica particles and from theliquid phase of the sol. This may be readily accomplished by treatingtypical silica sols of the type described in Bechtold et al., US. Patent2,547,902, with a cation exchange resin in the hydrogen form and astrong base anion exchange resin in hydroxide form. This treatment tendsto produce a finished aqueous sol in which both the continuous aqueousphase of the sol and the particles of silica are considered salt-free.Typical commercially available silica sols which may be deionized toproduce salt-free silica sols are those which are described in Table Iabove. These aqueous salt-free silica sols may be either used as such incombination with the silicic acid sol and with one or more of the namedrefractories to constitute a slurry coating material or may be modifiedwhereby the aqueous phase is completely or partially exchanged for ahydrophilic polar liquid such as an alcohol or the alcohol is mixed withaqueous sol in desired proportions. The salt-free pure alcohols oraqueousalcoholic silica sols may then be easily combined with silicicacid sols and a refractory and the resultant slurry used to coat thestools.

When the particle sizes of the silica sols described above are withinthe ranges specified, the silica particles present in the startingaqueous or organic sol have specific surface areas of at least 20 m./.g., and usually in excess of 100 mF/g. Further, when deionized solsare employed as a binder, they generally have a salt content expressedas Na SO of less than 0.01%.

The particular silicic acid sols used as a binder component may beproduced by a wide variety of methods. All of these particular sols haveaverage molecular weights below about 90,000. More preferably these acidsols contain silica particles having an average molecular weight of fromabout 1000 to 46,000. The pH of these acid sols is below 5.5 and morepreferably they lie within the range of 2.5 and 3.5. The averageparticle diameter is less than 5 millimicrons and generally 1-4millimicrons.

One method of preparing such acid sols is to neutralize water glass witha mineral acid. In using this method to form the acid silica sols it isnecessary, however,

to remove the major portion of the salts formed by neutralizationreaction. This may be accomplished by dialysis or electrodialysis, butthese procedures are not adaptable to large scale economic production.An improved method for preparing acid sols has been described in BirdU.S. Patent 2,244,325. By utilizing the teachings of this patent, thepreferred starting acid sols are produced. According to the Bird method,a water glass (alkaline silicate) solution is passed through a column ofcation exchange material in the hydrogen form whereby the alkali metalin the water glass is exchanged for hydrogen and the resultant productis an acid silica sol of unusual purity. Generally, the pH of the solsso produced lie within the range of 2.0-4.0. In addition, the averagemolecular weight of the silica particles is well below 90,000. Also,silicic acid sols generally have a Si0 solids content ranging from about2% to about 10%.

Other acid sols suitable for use in the invention may be prepared by avariation of the Bird method described above. In this embodiment theefiluent from the Bird process may then be further treated by passing itthrough a weak base resin in the free base form. The resultant productis then substantially stripped of any ions and is generally known asdeionized. Still another variation of the technique is to employ a mixedresin bed, that is, a bed containing a weak base resin in the free baseform and a strong acid resin in the hydrogen form whereby the silicicacid sol is formed simultaneously with exchange of its companion ions toproduce a substantially deionized polysilicic acid sol.

While the above described methods are preferable to produce the startingacid sol, it must be understood that any appropriate method forproducing an acid sol of a requisite molecular weight and pH may also beused. For example, minute amounts of the stabilizer such as alkali metalhydroxide may be used without departing from. the scope of the inventionas long as the pH is not raised above the operative limits describedabove.

The binder itself should be composed of 4090% colloidal silica sol and1060% silicic acid sol. More preferably, each component ranges fromabout 40% to about 60%. A 50%50% mixture was found to exhibit excellentperformance in its intended end-use. All expressed percentages areweight percents.

As mentioned above, the most preferred refractory materials are thosegenerally referred to as vitreous silicas. These are glassymodifications of silica, obtained by the fusion of selected lowtemperature crystalline forms, and are frequently referred to as quartzglass or silica glass. Specific vitreous silicas include those particlesmade fromfused quartz glasses, silicate glasses, silica glasses such asthe well-known Vycor materials and fused silica glasses. With respect toall of these materials the thermal expansion coefiicients are relativelysmall in proportion to other refractories such as those of the soda-limeand lead glass types. Generally, they have thermal expansioncoefficients smaller than 5x10" cm./cm./ C. Also, the silica content ofthese granular siliceous refractory materials is generally greater than96% silica expressed as SiO and may range as high as 99.8% SiO Thus, bythe term vitreous silica is meant a refractory comprising a silica glasshaving a thermal coefiicient of expansion and SiO content within theabove range.

It has been determined that for best results in coating stools therefractory used in the silica slurry should be able to withstand severeheat shocks. Due to the extreme hot temperature of the molten metal ascompared to that of the stool even when the latter is heated, anexceedingly abrupt change in temperature occurs when the metal contactsthe stool. The coating must itself be able to withstand this heat shockto impart necessary protection to the stool base. It has been theorizedthat failure of some prior art materials was due, at least in part, totheir inability to Withstand this sudden increase in heat, therebyresulting in cracking of the coating and subsequent exposure of themetal surface to the cascading molten metal poured into the mold.

In view of the above it is generally thought that the most preferredrefractories are those which have the highest purities concomitantwith'the lowest thermal coefficient of expansion. These properties areparticularly posit sessed by vitreous silicas and more particularlythoseof the fused silica types. The latter materials have a silicacontent greater than 97% silica expressed as SiO- and a thermalcoeflicient of expansion not greater than about 6X 10* cm./cm./ C.

A typical fused silica of the type described above which is extremelyuseful in the practice of the invention, having a thermal coefiicient ofexpansion of about 10 cn1./cn1./ C., has the following typical analysis:

TABLE II Ingredients: Percent by weight SiO 97.3 A1203 1.7 Sub-oxides ofsilica 1.0

The above type silica products are readily prepared by grinding verypure fused silica glasses. Likewise, the borosilicate glasses and Vycorsilica glasses may. be also ground to produce extremely usefulrefractories.

The particle size of the refractory may vary over a wide range. It ispreferred, however, that the refractory particles be sufiiciently smallso that a uniform dispersion of refractory and binder may be made. Thesmaller the particle size the longer a slurry made up of binder andrefractory, remains in a homogeneous state. It has been determined thatparticles ranging in size from 100 mesh to as low as a fraction of amicron may be employed. Preferred refractory materials have an averageparticle size ranging from a fraction of a micron to 500 microns inparticle diameter, with particles corresponding to the lower rangediameters being most preferred. Specific vitreous silica substances,marketed under the name, Nalcote fall within the above preferredparticle size range and have been employed with much success inpreventing.

erosion of base portions of molds and adherenceof same to the formedingots.

The amount of binder making up a portionof the coating slurry must besuch that it is present in an amount sufficient to bind the refractoryparticles together to thereby form a tightly adherent, continuous andunbroken coating which is securely bonded to the surface of the stool.Without proper amount of binder in relation to refractory, the resultantcoating, after application and drying of slurry, exhibits a pan-cakeeffect with nu-.

art processes. It is understood, of course, that these exam-ples aremerely illustrative, and that the invention is not limited thereto.

Example I In Order to particularly test the efliciency of the bindercomponent of the invention, a laboratory test was devised whichsimulated a commercial stool coating operation. Six binder test blendswere prepared having the following make-up: 90% silica sol-10% silicicacid sol;

80% silica sol-% silicic acid sol; 70% silica;sol%

silicic acid sol; 60% silica sol-% silicic acid Sol; silica sol-50%silicic acid sol and 40% silicia sol and silicic acid sol. The colloidalsilica sol contained 35% silica solids, and the silicic acid sol wasmadeup of 8 approximately 5% solids. The binders were applied to a castiron plate heated to about 325 F., and the coatings then inspected. Inall cases, the above described binder compositions gave a hard adherentcoating. Application of the colloidal sol alone also gave an acceptablecoating. However, it was surprising to note that the combination ofsilicic acid sol and colloidal silica sol with an overall solids contentof 17% was equal to or even slightly superior to the colloidal silicasol alone, having a solids content of 30%, or almost twice the solidscontent of the combination binder. Since the ultimate strength of thecoating is derived solely from the silica solids contained in the binderwith the liquid portion of the binder being volatilized off, it wasentirely unexpected that the above results would be forthcoming. It isbelieved however that the excellent results obtained via use of thebinders of the invention are attributable to the situation of arelatively wide range of silica particle sizes'donated by thecombination of binder ingredients.

Example II In this case, an actual mill trial was effected. A binder ofthe invention was formulated by mixing in equal weight proportions a 30%aqueous colloidal silica sol corresponding to silica sol N0. 2 whosephysical characteristics are tabulated in Table I, and a 5% aqueoussilicic acid sol prepared according to the teachings of Bird, U.S.Patent 2,244,325. A silica slurry was made by addition of equal part ofthe above binder and fused silica refractory. This latter material has awide range particle size distribution in which 100% will pass a 100'mesh sieve, will pass 325 mesh, 30% is smaller than 10 microns and thesmallest particles are a fraction of a micron in size. The refractoryand binder were thoroughly mixed and then coated onto a number of castiron stools. The coatings had excellent adherence, and surface hardnessand as well, were easy to apply. In comparison runs, a 30% silica sol incombination with fused silica, also at a 1:1 weight ratio, also yieldeda coating having the above described excellent characteristics. However,in some instances employment of the binder composition of the inventionyielded an even more adherent, coating than a binder containing only thesilica sol component. Again, such results were surprising and unexpectedin view of the fact that the solids content of the combination binder ofthe invention was almost one-half that of a binder containing silica solas the sole ingredient.

The coatings derived from the'slurries of the invention and containing abinder of the type described herein substantially prevented erosion ofthe stools or base members of the metal molds during pouring of moltenmetal thereon. Moreover, adherence of the subsequently formed ingots tothe base member after metal solidification was also substantiallyreduced. In addition, subsidiary good effects were also noted. Forexample, since erosion has been reduced to a minimal effect, life ofmold bases is substantially'increased. Likewise, since generally noadherence of the mold base portions to the ingots occurs, no resort needbe taken to physicallyxcontacting the two adherent articles against athird larger object such as a floor, to separate the united objects andrecover the ingot. As mentioned above, not only is this separationprocedure time-consuming and costly, but it frequently results incracking or complete disintegration of the stool, making it completelyunsuitable for further use. Also, since little or no erosion occursfromv the molten metal, the amount of metal required to be cropped fromthe ingot is substantially reduced. As an overall advantage, the moldinventory necessary for efiicient operation may be substantially reducedthrough use of the slurries of the invention.

Other advantages are to be noted. For example, sincesemi-permanentcoatings on stools may be maintained by formation of thecoating film, morepductile stools may 9 be utilized. Also, lighterstools may be employed, cutting down on track weight and trackmaintenance, since stools are normally carried by means of flat cars onrails to the point of pouring of metal. Again, since the coating appearsto withstand a wide range of temperatures, it is possible that severetemperature surface applications may be performed which were heretoforeimpossible due to failure of prior art coatings. Many other advantagesof the mold forming process of the invention are apparent.

It is understood, of course, that the binders of the invention may beutilized with other known binders such as ethyl silicate, aluminumphosphate, sodium silicate, etc.

The invention is hereby claimed as follows:

1. An improved metal mold for casting metal ingots which comprises anopen-top mold having at least its base member coated with a thin solidprotective film of refractory material derived from drying a slurrycomprising at least one refractory selected from the group consisting ofvitreous silica, crystalline silica, aluminum silicate, alumina,graphite, zirconium silicate, magnesium silicate, and clay suspended ina binder comprising a colloidal silica sol and a silicic acid sol, saidbinder being present in said slurry in an amount sufficient to bind therefractory materials together to thereby form a tightly adherent coatingwhich is bonded to said base member, said coated base member beingfurther characterized as being nonerosive to flowing molten metal andnon-adherent to subsequently formed solid metal ingots.

2. The metal mold of claim 1 wherein said coating is derived from aslurry comprising 10-70 parts by weight of vitreous silica having asilica content not less than 96% silica, expressed as S102, and athermal coeflicient of expansion of less than 5X 10- cm./cm./ C.; and30-90 parts by weight of said binder.

References Cited UNITED STATES PATENTS 2,701,902 2/ 1955 Strachan106-38.3 X 2,806,270 9/1957 Shaul 10638.3 X 2,842,444 7/1958 Embleml0638.3 3,035,318 5/1962 Campbell 164-72 3,059,296 10/1962 North10638.27 X 3,184,813 5/ 1965 OShea 164-72 3,184,815 5/1965 Reuter 164723,209,421 10/ 1965' Shepherd 106 38.3 X 3,234,607 2/1966 Hammerlund106-38.3 X 3,303,030 2/1967 Preston 10638.9 X 3,314,117 4/1967 Fishmanet a1. 106-383 X J. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner.

